The bipolar junction transistor (BJT) differential pair is often used as the key element in a mixer cell. The input signal is a current applied to the emitters of the differential pair of transistors. A differential voltage applied between the bases switch the transistors so that one is turned on while the other is off and vice-versa. The output signal is the collector current or the differential collector current of both of the transistors. The output current includes mixing products that contain frequency components at the sum and difference frequencies between the input emitter current and the differential voltage applied to the bases. For radio frequency (RF) mixers, the differential voltage signal applied at the bases is usually a local oscillator (LO) signal.
The ideal differential voltage applied to the bases of differential pair mixer cells is just large enough to switch one of the pair off (but no larger) with very fast transitions, i.e., short rise and fall switching times. Excessively large voltage levels of the LO signal, however, result in common mode currents at a frequency of twice the LO frequency, which is not desirable. Switching times that are too slow increase the mixer cell noise figure, which is also not desirable.
The ideal LO mixer driver would generate a square wave with a peak-to-peak voltage of 4VT to 8VT, where VT is a thermal coefficient voltage (the voltage equivalent of temperature). In particular, VT=kT/q, where xe2x80x9ckxe2x80x9d is the Boltzmann constant in joules per degree Kelvin, T is the temperature in degrees Kelvin (absolute scale), and xe2x80x9cqxe2x80x9d is the magnitude of the charge of an electron. Resistively loaded differential pairs form a type of limiting amplifier that is often used. In a particular embodiment, for example, the differential pair includes a pair of NPN bipolar junction transistors, Q1 and Q2, having their emitters coupled together and to a current sink that sinks a bias current IBIAS1 to ground. The collector of each of the transistors Q1 and Q2 is coupled to one end of a respective one of two load resistors R1 and R2. The other ends of the resistors are coupled to a supply voltage VSupply. A low level LO signal, such as a sine wave or the like, applied between the bases of the differential pair switches one transistor on and the other transistor off. The xe2x80x9coffxe2x80x9d transistor""s collector is pulled up to the supply voltage. The xe2x80x9conxe2x80x9d transistor""s collector voltage is below the supply voltage by an amount equal to the current/resistance (IR) drop across the corresponding load resistor. Therefore, the amplitude of the resulting square wave is 2IR, where I is the value of the current sink (IBIAS1) biasing the differential pair and R is the value of one of the collector load resistors (R1 or R2).
The mixer can be driven by the collector voltages, VOUT+ and VOUTxe2x88x92 of the transistors Q1 and Q2, respectively. The input to the mixer cell is capacitive. The switching times, therefore, are limited by the current available to charge and discharge the input capacitance. With the differential pair completely switched, the positive going output has net output drive current limited by the resistive value of the collector load resistors. The negative going output has net output drive current, which is initially equal to the current of the current sink. As soon as the voltage begins to drop, however, the negative going output is equal to the difference between the current sink current and the collector load resistor current. To increase the switching times, the resistors had to be smaller and the current sink current had to be larger.
A modification of the above-described mixer driver is the addition of a pair of emitter follower NPN bipolar junction transistors Q3 and Q4 having their respective bases connected to respective collectors of the differential pair Q1, Q2. The collectors of the transistors Q3 and Q4 are coupled to VSupply and each emitter is coupled to a respective constant current sink drawing a constant bias current IBIAS2 to ground. These flat emitter followers, therefore, have constant current sink biasing. The voltage swing is still set by the resistor IR drop. The emitter followers can supply a lot of current to pull the mixer cell input positive. The resistors may still limit the switching speed, however, because the emitter follower base capacitance has to be charged up before these followers can supply excess current. Also, for the negative going output, the maximum current available to drive the mixer cell is the emitter follower current sink current (IBIAS2).
A mixer driver circuit according to an embodiment of the present invention includes a differential pair, a differential current supply and a switched current sink. The a differential pair has a differential input for receiving a differential input signal. The differential current supply provides a differential output and switches in response to switching of the differential pair to provide a differential output current. The switched current sink biases the differential current supply and sinks current to drive the differential output signal.
The differential pair may be a resistive-loaded differential pair of transistors biased by a constant current sink. The differential current supply may include a pair of emitter follower buffers. The mixer driver may further include a pair of constant current sinks and a second pair of emitter follower buffers, where the second pair of emitter follower buffers is biased by the pair of constant current sinks and provides a voltage level shifting drive for the switched current sink. The switched current sink may be biased by a constant current source and have a pair of control inputs coupled to respective polarities of the differential output. Alternatively, the control inputs of the switched current sink are coupled to respective polarities of the differential input of the differential pair.
The mixer driver circuit may further include a second differential current supply that operates to switch the switched current sink based on switching of the differential pair. In one embodiment, the switched current sink has first and second control inputs, and the second differential current supply includes a pair of emitter follower buffers. Each of the emitter follower buffers is biased by a constant current source, has a control input driven by the differential pair and has a current output for driving a respective control input of the switched current sink.
In a more specific embodiment, the differential pair includes first and second bipolar junction transistors (BJTs). The bases of the differential pair of transistors receive first and second polarities, respectively, of a differential input signal. The mixer driver circuit includes a first current sink, first and second bias resistors, third and fourth BJTs and a switched current sink. The first current sink is coupled between the common emitters of the first and second BJTs and a reference signal. The first and second resistors are each coupled between a voltage source and a collector of a corresponding one of the first and second BJTs. The third BJT has a collector coupled to the voltage source, a base coupled to the collector of the first BJT and an emitter for providing a first polarity of a differential output signal. The fourth BJT has a collector coupled to the voltage source, a base coupled to the collector of the second BJT and an emitter for providing a second polarity of the differential output signal. The switched current sink is coupled to the emitters of the third and fourth BJTs, biases the third and fourth BJTs relative to the reference signal and sinks current to drive the differential output signal.
In one embodiment, the switched current sink includes fifth and sixth BJTs and a second current sink. The fifth BJT has a collector coupled to the emitter of the fourth BJT and a base coupled to the emitter of the third BJT. The sixth BJT has a collector coupled to the emitter of the third BJT, a base coupled to the emitter of the fourth BJT and an emitter coupled to the emitter of the fifth BJT. The second current sink is coupled between the common emitters of the fifth and sixth BJTs and the reference signal. In an alternative embodiment, the switched current sink includes substantially the same components except that the base of the fifth BJT is coupled to the base of the first BJT and the base of the sixth BJT is coupled to the base of the second BJT.
In yet another alternative embodiment, the mixer driver circuit may further include a third resistor having a first end coupled to the second end of the first resistor and a second end coupled to the collector of the first BJT, and a fourth resistor having a first end coupled to the second end of the second resistor and a second end coupled to the collector of the second BJT. The mixer driver circuit may further include a fifth BJT having a collector coupled to the voltage source and a base coupled to the collector of the first BJT, and a second current sink coupled between the emitter of the fifth BJT and the reference signal. The mixer driver circuit may further include sixth BJT having a collector coupled to the voltage source and a base coupled to the collector of the second BJT, and a third current sink coupled between the emitter of the sixth BJT and the reference signal. The switched current sink may include a seventh BJT having a collector coupled to the emitter of the fourth BJT and a base coupled to the emitter of the fifth BJT, and an eighth BJT having a collector coupled to the emitter of the third BJT, a base coupled to the emitter of the sixth BJT and an emitter coupled to the emitter of the seventh BJT. Also, a fourth current sink is coupled between the common emitters of the seventh and eighth BJTs and the reference signal. In any of these embodiments, the reference signal may be ground.
A mixer driver circuit as described herein switches a capacitive load, such as a mixer cell or the like, faster for a given supply current because the output is a push-pull type arrangement which supplies a net output current quicker. The loading at the input differential pair is also reduced.